Electronic ballasts typically include an inverter that provides high frequency current for efficiently powering gas discharge lamps. Inverters are generally classified according to their topology (e.g., half-bridge or push-pull) and the method used to control commutation of the inverter switches (e.g., driven or self-oscillating). In many types of electronic ballasts, the inverter is a driven half-bridge in which two alternately conducting power transistors are used to provide a square wave output voltage. The square wave output voltage of the inverter is processed by a resonant output circuit that provides high voltage for igniting the lamps, as well as a magnitude-limited current for powering the lamps.
When the lamps fail, are removed, or otherwise cease to operate in a normal fashion, it is highly desirable that the inverter be shut down or shifted to a different mode of operation. This is necessary in order to minimize power dissipation, reduce heating in the ballast, and protect the inverter transistors from damage due to excessive voltage, current, and heat. Circuits that shut down or alter the operation of the inverter in response to lamp removal or failure are customarily referred to as inverter protection circuits.
One known type of inverter protection circuit monitors the resonant output circuit for overvoltage and correspondingly turns the inverter driver circuit off if the voltage exceeds a certain level. This approach is not attractive for those ballasts in which the lamps are "direct coupled" in series with the resonant circuit, since removal or failure of the lamps actually opens the resonant circuit and thereby prevents the development of high voltage in the resonant circuit. However, even with the resonant circuit open, the inverter continues to operate, resulting in unnecessary power dissipation in the inverter switches.
Another known approach utilizes a current path through the lamp filaments to detect lamp removal or failure. This approach alone is inadequate for those situations in which a lamp fails to operate in a normal manner, but its filaments remain intact, such as what occurs with "degassed" and "diode mode" lamps. Furthermore, if multiple lamps are present, and if a single current path through the filaments of all the lamps is used, the inverter is shut down even if only one filament of one lamp fails but the remaining lamps are operational and with their filaments intact. This is unnecessary and undesirable, since it is preferred to have the inverter continue to operate so that the remaining operational lamps may continue to provide illumination, thus obviating any urgent need for replacement of the single failed lamp.
It is therefore apparent that a need exists for an inverter protection circuit for a driven half-bridge type inverter that provides protection of the inverter switches and other ballast components under various lamp failure modes, such as lamp removal or a degassed lamp, but that allows the inverter to continue to operate when at least one operational lamp is present and when the failed lamps present no danger to the inverter. Such a circuit would represent a considerable advance over the prior art.